RFR: Remove C2 leftovers

[Roman Kennke]

- a bunch of lone includes
- a !UseShenandoahGC that was added when we wanted barriers on
constants, which we don't do anymore.
- cast_to_nonconst() which I believe was also added for supporting
barriers on constants (and there was meant to prevent replacing a
barrier with it's underlying constant). I don't think we need this
anymore, and it might actually be harmful. We ensure that constants are
always to-space.

It passes tier3_gc_shenandoah

Ok?

Roman